docs: Update 2.0 migration guide to include confusing/scary warning about discovery.yml not being parsed

As reported in https://meltano.slack.com/archives/C01TCRBBJD7/p1654785151254499:

I wanted to hold off the upgrade to Meltano 2.0 a bit in case there were some small issues, but now I’m getting The discovery.yml manifest received from https://discovery.meltano.com/discovery.yml could not be parsed. errors so I guess I have no choice.

It’s just a warning, and Meltano will fall back on the cached or bundled copy, but it still makes people seem like something’s wrong. We can’t go back and make the warning message sound less concerning, of course.

One option would be to roll https://discovery.meltano.com/discovery.yml back to the latest version supported by <2.0 (version 20, iirc), since 2.0 will use the Hub anyway. 2.0 will find an outdated copy at the location, and fall back on its own bundled version, which is OK. It will also show a warning about the remote discovery.yml, but there are fewer users on 2.0 than on pre-2.0 affected by that, and we can suppress that in 2.0.x and only use the bundled copy, since the remote discovery.yml won’t be updated anymore anyway and the Hub API is the preferred way to look up plugin definitions going forward.

[edgarrmondragon]

@tayloramurphy yes, the warning caused by Ruamel dumping and the trying to parse back discovery.yml should be gone now.

[tayloramurphy]

@aaronsteers since this is now a documentation issue, are you okay if we move it to next week’s iteration? The scope is:

• Add a new FAQ section on the migration guide
• Add this particular error / warning as a question with an appropriate answer